Method of producing a phosphor dot screen for a color picture tube by an electron beam printing



SADAO OZAKI ETA 3,527,652

Sept. 8, 1970 METHOD OF PRODUCING A PHOSPHQR DOT SCREEN FOR A COLOR PICTURE TUBE BY AN ELECTRON BEAM PRINTING 1968 4 Sheets-Sheet 1 Filed Feb. 12,

Sept. 8, 1970 SADAQ QZAKI ETAL 3,527,652

METHOD OF PRODUCING A PHOSPHOR DOT SCREEN FOR A COLOR,

PICTURE TUBE BY AN ELECTRON BEAM PRINTING Filed Feb. 12, 19 8 4 Sheets-Sheet 2 24 igi i 26 Sept. 8, 1970 SADAQ QZAKI ETAL 3,527,652 METHOD OF PRODUCING A PHOSPHOR DOT SCREEN FOR A COLOR PICTURE TUBE BY AN ELECTRON BEAM PRINTING Filed Feb. 12, 1968 4 Sheets-Sheet 5 SADAO OZAKI ET AL 3,527,652

Sept, 8, 1970 METHOD OF PRODUCING A PHOSPHOR DOT SCREEN FOR A COLOR PICTURE TUBE BY AN ELECTRON BEAM PRINTING Filed Feb. 12, 1968 4 Sheets-Sheet United States Patent "ice US. Cl. 117-335 12 Claims ABSTRACT OF THE DISCLOSURE A method of producing a phosphor dot screen for a color picture tube which consists primarily in producing a revolving magnetic field or a revolving electric field between deflecting coils and electron guns. The electron beams produced by said electron guns can be slightly deflected by said revolving magnetic field or revolving electric field before being deflected by said reflecting coils and passing through apertures of a shadow mask to be converged on a coat of phosphor material applied to a face plate. In this way, a phosphor dot screen having phosphor dots larger in size than the spots of electron beams can be produced.

This invention relates to methods of producing a phosphor dot screen for a color picture tube, and concerns in particular with a method of producing a phosphor dot screen for a color picture tube by electron beam printing.

In general, color picture tubes for reproducing images in color include a face plate on which are disposed two or more kinds of phosphor dots arranged in a geometrical pattern. Said phosphor dots emit light of different colors in response to incident electron beams and provide images in different colors in accordance with the information signals transmitted. The phosphor dots formed on the face plate of the color picture tube should be arranged accurately and precisely in order to preclude faulty color renderings and contamination of color when electron beams are incident on the phosphor dots.

In shadow-mask color picture tubes which are used as color picture tubes of the after acceleration and convergence type, it has hitherto been customary to print and form phosphor dots in the manner described hereunder.

A transparent conducting electrode, such as NESA, for example, is first mounted on the inner surface of a face plate of a color picture tube, and then a coat of a mixture of one of red, blue and green phosphor materials or a blue phosphor material, for example, and a sensitive material is applied to said transparent conducting electrode. A first voltage V is applied to the transparent conducting electrode in a vacuum, and a second voltage V (V V is applied to a shadow mask so as to create an accelerating electric field for increasing the velocity of electrons in electron beams between said coat of the mixture of blue phosphor material and sensitive material and the shadow mask. The electron beam from the blue electron gun which has passed through apertures in the shadow mask is caused by said accelerating electric field to converge on the coat of the mixture of blue phosphor material and sensitive material upon the scanning of a raster on the coat of the mixture of blue phosphor material and sensitive material, so that the electron beam prints dots on the coat which are smaller in size than the apertures of the shadow mask. Thus, only the dots 3,527,652 Patented Sept. 8, 1970 printed by electron beam are sensitized and formed on the transparent conducting electrode.

Then, the face plate is removed from the color picture tube and unsensitized portions of the coat of the mixture of blue phosphor material and sensitive material are washed off to thereby leave blue phosphor dots. Green and red phosphor dots can be printed and formed in the same manner by using corresponding electron guns. It is thus possible to form a set of three phosphor dots of different colors or red, blue and green phosphor dots for each aperture of the shadow mask.

The method described of the prior art has had disadvantages. The phosphor dots formed by the method described are substantially equal in size to the spots of electron beams incident on the coat of phosphor. In the method described, it is impossible to form phosphor dots which are larger in size than the thickness of electron beams. When a color picture tube equipped with a phosphor dot screen produced in the manner described is used to provide color pictures by means of electron beams identical with the electron beams used for producing the phosphor dots as described above, the three electron beams will be prevented from centering properly on the phosphor dots if there is a slight displacement in the position of deflecting coils or a variation in the voltage applied to the picture tube electrode, since such aberration will cause the paths of travel of electron beams to be deviated from those of the electron beams used for forming phosphor dots. This prevents electron beams from falling on the entire areas of phosphor dots, leaving parts of the phosphor dot areas unlighted. It thus becomes impossible to achieve high color purity and good convergence.

Electron beam spots can be varied in size by properly changing the ratio of the voltage applied to the shadow mask to the voltage applied to NESA or other transparent conducting electrode. This causes no aberrations in portions of the phosphor dot screen in the vicinity of the center axis of color picture tube. However, since the electrode beams deflected by the deflecting coils strike the phopshor dot screen obliquely with respect to the surface of shadow mask in other portions of the phosphor dot screen, the index of refraction of electron beams in an electric field between the shadow mask and the electron guns will vary from that in an electric field between the shadow mask and the phosphor dot screen. For this reason, a change in the voltage ratio as aforementioned will result in the electron beams striking the phosphor dots at positions different from those at which the beams struck the phosphor screen in printing and forming phosphor dots. Color images will thus be reproduced by electron beams differing in conditions from the electron beams used for forming phosphor dots. This also causes a deterioration in convergence and color purity.

The present invention obviates the aforementioned disadvantages of the prior art. According to the invention, there is provided a method of producing a phosphor dot screen for a color picture tube by electron beam printing which permits to provide color pictures of markedly improved color purity and brightness by forming phosphor dots which are larger in size than electron beam spots.

Accordingly, one object of the present invention is to provide a method of producing a phosphor dot screen for a color picture tube which permits to readily achieve good convergence.

Another object of the invention is to provide a method of producing a phosphor dot screen for a color picture tube which permits to readily achieve high color purity.

Still another object of the invention is to provide a method of producing a phosphor dot screen for a color picture tube which permits to provide color pictures of high brightness.

According to the method of this invention, a revolving magnetic field or a revolving electric field is created between deflecting coils and electron guns of a color picture tube. The electron beam produced by one of the electron guns is slightly deflected by this revolving magnetic field or revolving electric field while it is passing therethrough. Then, the electron beam is greatly deflected by the magnetic field produced by the deflecting coils in passing through apertures of a shadow mask to impinge on a face plate having a coat of phosphor material thereon and print spots which are identical in size to the spots of the electron beam. Since the aforementioned revolving magnetic field or revolving electric field changes its direction with time, a portion of electron beam produced by the electron guns later than the preceding portion of electron beam by a time interval corresponding to one frame period is slightly deflected by the revolving magnetic or electric field in a direction different from the direction in which said preceding portion of electron beam is deflected. The first mentioned portion of electron beam passes through the same apertures of shadow mask as the last mentioned portion of electron beam, but the direction of incidence of the former varies from that of the latter. Accordingly, the first mentioned portion of electron beam strikes the coat of phosphor on the face plate and print spots at positions different from the positions at which the last mentioned portion of electron beam strikes the coat of phosphor and print spots. From the foregoing, it will be apparent that if the electron beam is continuously produced for several frame periods, it is possible to print phosphor dots on the face plate each of which consists of a conglomeration of electron beam spots and is larger than the electron beam spots in size.

By repeating the aforementioned operation successively three times for red, green and blue colors, a phosphor dot screen with phosphor dots of red, green and blue colors formed thereon can be produced. If a color picture tube incorporating the phosphor dot screen produced by the method of this invention is employed for reception of images in color by using the same electron beams as the electron beams used for printing and forming phosphor dots and by applying the same voltage to the tube electrode as the voltage applied in printing and forming phosphor dots, adjustments of convergence will be greatly facilitated and the picture provided will have markedly high color purity and increased brightness.

Other objects and advantages of the invention will become apparent from consideration of the following description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional side view of a color television tube provided with a face plate on which phosphor dots are printed and formed by the method according to this invention;

FIG. 2 is a view in explanation of the refraction of an electron beam caused by an after accelerating electric field;

FIG. 3 is a view in explanation of the manner in which an electron beam is caused, after passing through a shadow mask, to converge on a coat of phosphor by the after accelerating electric field;

FIG. 4 is a perspective view showing the manner of convergence of electron beams in the color television tube of FIG. 1;

FIG. 5 is a longitudinal sectional side view showing one embodiment of the method according to this invention;

FIG. 6 is a view in explanation of one example of essential portions of the embodiment of FIG. 5;

FIG. 7 is a diagram showing a circuit for generating an electric current to be passed to a coil of FIG. 6 according to the method of this invention;

FIG. 8 is a diagram showing another circuit for generating an electric current according to this invention;

FIG. 9 is a view in explanation of the relation between an electron beam and a coat of phosphor on a face plate when an image is received;

FIG. 10 is a view in explanation of the manner in which a phosphor dot is formed on a face plate of a color television tube by the method according to this invention;

FIG. 11 is a view in explanation of the relation between a phosphor dot formed by the method according to this invention and an electron beam spot;

FIG. 12 is a diagram showing the path of travel of electron beams in a color picture tube in which no after accelerating electric field exists;

FIG. 13 is a view in explanation of the manner in which an electron beam strikes a coat of phosphor on a face plate after passing through an aperture of a shadow mask in a color picture tube in which no accelerating electric field exists;

FIG. 14 is a view in explanation of the relation between an electron beam and phosphor dots when the method according to this invention is carried into practice with a color picture tube in which no accelerating electric field exists; and

FIG. 15 is a view showing an arrangement of electron guns in a color picture tube to which the method according to this invention can be applied.

For better and fuller understanding of the method according to this invention, it is necessary to give an outline of the operation of a color picture tube with which the method can be carried into practice. FIG. 1 shows one example of color picture tube to which method according to this invention can be applied. 11 is an outer casing formed as of glass; 12 designates deflecting coils; 13 designates three electron guns which, though represented by a single block, are arranged circumaxially about the tube axis and disposed equiangularly at an angle of 14 is a base; 15 is a face plate of glass which is bonded to the outer casing 11 at 16; 17 is a transparent conducting electrode, such as NESA, for example, which is mounted on the surface of the face plate which faces toward the electron gun assembly (see FIG. 4); 18 is a phosphor dot screen produced by the method of electron beam printing according to this invention as subsequently to be described, on which red (R), blue (B) and green (G) phosphor dots are disposed; 19 is a metal back; 20 is a shadow mask formed with a multitudes of apertures 21, one aperture for each set of three phosphor dots of red, blue and green colors (see FIG. 4); 22 designates three electron beams each produced by one of the three electron guns 13; 23 is an anode of conducting material applied to the inner surface of the glass outer casing 11; and 24 is a first power source for supplying a voltage V to the metal back 19 and transparent conducting electrode 17, 25 is a second power source for supplying a voltage V to the shadow mask 20, and 26 is a third power source for supplying to the anode 23 a voltage V which is slightly higher than the voltage V supplied by said second power source. In one example of carrying the method of this invention into practice, V is 20 kv., V 6.7 kv. and V 7 to 10 kv.

Upon actuating the color picture tube by applying various voltages, an accelerating electric field is created between the phosphor dot screen '18 and the shadow mask 20. Because of the fact that there is a difference in intensity between the electric fields on opposite sides of the shadow mask, the paths of travel of the electron beams are refracted when the beams pass through an aperture 21 of the shadow mask. Specifically, as shown in FIG. 2 (which shows a portion of phosphor dot screen far removed from the center axis of tube), the electron beam B incident on the center axis of the aperture 21 at an angle of a is caused, after passing through the aperture 21, by the accelerating electric field to advance at an angle u) and strike the phosphor dot screen 18. Each electron beam 22 is accelerated and converged as shown in FIG. 3, and is reduced into a small spot as shown at 27 when it falls on the phosphor dot screen. Therefore, in a color picture tube provided with three electron guns as shown in FIG. 1, three electron beams passing through the same apertures 21 of the shadow mask are converged and strike the corresponding three phosphor dots which emit light of red, blue and green colors.

The voltage V applied by a third power source 26 is slightly higher than the voltage V of a second power source in FIG. 1 as aforementioned. This arrangement permits the anode 23 to absorb secondary electrons produced when the electron beams 22 impinge on a portion of the shadow mask 20 around the edge of the apertures 21 in passing therethrough, which might otherwise be accelerated by the after accelerating electric field and move toward the phosphor dot screen 18 to excite the phosphor dot screen 18 to emit unnecessary light, thereby reducing color purity of the picture.

In the color picture tube shown in FIG. 1, there exists between the phosphor dot screen 18 and the shadow mask 20 an accelerating electric field produced by a difference (V V in the voltages supplied by first and second power sources 24 and 25. The electron beam is refracted as shown in FIG. 2. This makes it impossible to produce the phosphor dot screen of a color picture tube of this type by an optical printing method relying on a conventional light source. Accordingly, it has hitherto been customary to use an electron beam printing method as described in the opening paragraph of this specification. As can be seen from FIG. 1, however, the electron beams produced by the electron guns are deflected only by the deflecting coils, and the deflected electron beams passing through the apertures of the shadow mask are converged before striking the screen. The phosphor dots printed by the electron beams are not larger than the thickness of electron beams, and consequently the phosphor dots are substantially as large as the spots of electron beams. This causes aforementioned disadvantages to present themselves.

FIG. 5 is a view in explanation of one embodiment of the method according to this invention. The color picture tube illustrated in FIG. 5 is similar to the color picture tube shown in FIG. 1, so that like reference numerals designate similar parts. In FIG. 5, 28 designates convergence coils which are omitted in FIG. 1 because said coils have no particular importance in the method according to this invention.

Some of the outstanding characteristics which distinguish the tube of FIG. 5 over the tube of FIG. 1 will be summarized in brief as follows:

(1) The tube of FIG. 5 includes a face plate 15 which is connected to an outer casing 16 through a vacuum packing (not shown);

(2) The tube of FIG. 5 has no metal back 19;

(3) The tube of FIG. 5 is provided with coils 29 for producing a revolving magnetic field, which are presently to be described, disposed between deflecting coils 12 and convergence coils 28; and

(4) A phosphor coat 18 applied to the electrode 17 is of a single color such as blue, for example.

One embodiment of coils 29 for producing a revolving magnetic field is shown in detail in FIG. 6, in which the coils 29 are enclosed by dotted lines. 30 is an annular magnetic core formed as of Permalloy (the core in this embodiment having an inner diameter of 5 3 millimeters, an outer diameter of millimeters, and a thickness of 10 millimeters). L designates coils would on the right and left portions of the core 30 and disposed in face to face relation with each other, while L designates coils wound on the upper and lower portions of the core 30 and disposed in face to face relation with each other, said coils L being angularly displaced from the coils L by 90 with respect to the center of annular core. The coils L and L have the same number of turns ,uh. for inductance and 0.35 52 for resistance in this embodiment). It will be apparent that if sine wave form currents of identical amplitude which differ from each other by in phase are passed to the coils L and L respectively, a revolving magnetic field can be produced inside the annular core 30. To attain this end, a voltage of E cos wt is applied to the coils L and a voltage of E sin wt is apllied to the coils L One example of circuit for generating the voltage E cos wt and E sin wt is illustrated in FIG. 7, in which a coil L is connected in series with a resistor R (having a resistance value582) and a coil L is connected in se ries with a capacitor C (having a capacitance600 f). These series components in turn are connected in parallel with each other, and then connected to a secondary winding of a transformer T. A sine wave voltage E sin wt is applied to a primary winding of the transformer T. The frequency corresponding, to to here may be of any value as desired so long as it is not equal to repeating frequencies of the horizontal synchronizing signal and vertical synchronizing signal of television of frequencies of values which are obtained by multiplying said repeating frequencies by an integer. In this embodiment, the frequency selected is 50 c./s., and the transformer T used is such that a voltage of volts is applied to its primary winding and a voltage of about 1 volt appears in its secondary terminal.

Impedances for the resistance value of aforementioned resistor R and the frequency 50 c./s. of aforementioned capacitor C are larger in value then impedances for the frequency of 50 c./s. of the coils L and L so that the values of currents flowing to the coils L and L are substantially determined by the impedances of the resistor R and the capacitor C respectively and the currents differ from each other in phase by about 90. It will be evident, therefore, that by properly adjusting the resistance value of the resistor R and the capacitance of the capacity C, it is possible to make the amplitude of current flowing to the coil L equal to the amplitude of current flowing to the coil L thereby producing a revolving magnetic field substantially circular in shape inside the annular core 30.

Alternatively, a revolving electric field can be used in the method according to this invention. A circuit as shown in FIG. 8 may be used for producing this revolving electric field. When the circuit shown in FIG. 8 is used, the coils 29 for revolving the electron beams are replaced by two sets of deflecting plates 31, 32 and 33, 34 of conducting material in the color picture tube shown in FIG. 5. Said deflecting plates 31, 32 and 33, 34 are disposed in face to face relation with each other as shown in FIG. 8. The circuit illustrated in FIG. 8 includes resistors r and r in place of the coils L and L of the circuit shown in FIG. 7, said resistor r being connected at its opposite ends to the deflecting plates 31 and 32 by wires and said resistor r being connected at its opposite ends to the deflecting plates 33 and 34 by wires. The resistor R has a resistance value which is substantially larger than the resistance value of the resistor r while the impedance for the frequency of 50 c./ s. of the capacitor C is made substantially larger than the resistance value of the resistor r By this arrangement, the values of currents flowing to the resistors r and r can be determined substantially by impedances of the resistor R and capacitor C, with said currents differing from each other in phase by about 90. It will be evident, therefore, that by properly adjusting the resistance value of resistor R and the capacitance of capacitor C, it is possible to make the amplitude of current flowing to the resistor r equal to the amplitude of current flowing to the resistor r Accordingly, reductions in voltage occurring at opposite ends of the resistors r and r are of equal amplitude and differ from each other in phase by 90, and a revolving electric field substantially circular in form and surrounded by the deflecting plates 31, 32, 33 and 34 can be produced in the vicinity of the center axis of the color picture tube.

It will be evident that when there exists a revolving magnetic field or a revolving electric field in a certain portion of a color picture tube on the center axis of the tube, electron beams passing through said portion are deflected by said revolving magnetic field or revolving electric field. Since the revolving magnetic field and the revolving electric field act in the same way on electron beams, the method according to this invention will hereafter he explained with reference to the rotary magnetic field.

A transparent conducting electrode, such as NESA, for example, is mounted on the inner surface of the face plate 15, and a coat of a mixture of one of the phosphor materials of red, blue and green or blue phosphor mate rial, for example, and a sensitive material is applied to said transparent conducting electrode. After mounting the shadow mask on the face plate 15, the face plate 15 is mounted on the outer casing 11 along the edge 16 through a vacuum packing as shown in FIG. 5, and the portion of the tube enclosed by the face plate 15 and the outer casing 11 is evacuated to form a vacuum therein (the vacuum packing and means for evacuating the portion described are not shown because they are publicly known).

Then, the coils 29 for producing a revolving magnetic field between deflecting coils 12 and convergence coils 28 as shown in FIG. 5, and the voltages described above are applied from direct current power sources 24, and 26 to the corresponding electrodes in the color picture tube as illustrated. In the figure, 22a, 22b and 22c designate electron beams produced by three electron guns 13 and demodulated by red, blue and green signals respectively. Assuming that control grid voltages applied to the electron guns are adjusted such that the electron beam 22a alone is produced and no frequency modulation signals are applied to the electron gun for blue color, the electron beam for blue color 22a produced by the electron gun 13 will be deflected by the revolving magnetic field produced by coils 29 while passing therethrough, and then greatly deflected by the magnetic field produced by the deflecting coils 12 through which flows a standard television current for effecting horizontal and vertical scanning. The electron beam for blue color 22a will pass through the apertures 21 of the shadow mask 20 and impinge on the coat of blue phosphor material. Blue phosphor spots are thus printed on the face plate 15 at positions 27 on which the electron beam 22a falls on the blue phosphor coat. Since said revolving magnetic field changes its direction with time, a portion of electron beam 22a produced by the electron gun 13 with a time lag exactly corresponding to one frame period will be deflected by the revolving magnetic field in a direction different from the direction of deflection of the preceding portion of the electron beam 22a in passing through the same apertures 21 of the shadow mask 20. The direction of incidence of the portion of electron beam 22a in the apertures is different, as shown in FIG. 9, from the direction of incidence of the portion of electron beam 22a, so that the portion of electron beam 22a' impinges on the blue phosphor coat at positions 27' different from the positions 27. Blue phosphor spots are thus formed on the face plate 15 at positions 27. It will be evident from the foregoing that if the electron beam is continuously produced for an interval corresponding to several frame periods, large phosphor dots each consisting of a conglomeration of several phosphor spots can be formed, as illustrated in FIG. 10, on the face plate 15. Specifically, the portions of electron beam 22a, 22a produced by the electron gun 13 pass through the same apertures 21 of the shadow mask 20 and are converged conically while successively changing the directions of incidence and yet maintained at a proper angle of incidence. It will be appreciated that each of the phosphor dots 35 printed by the portions of electron beams 22a, 22a which are accelerated and converged after passing through the shadow mask is much larger than the electron beam spot 27 as shown in FIG. 11.

After the blue phosphor dots are printed on the face plate, the evacuating means is rendered inoperative so as to restore atmospheric pressure to the tube and the face plate 15 is removed from the tube. The face plate 15 from which the shadow mask 20 is removed is treated so as to wash off unsensitized portions of the blue phosphor coat, thereby leaving blue phosphor dots on the face plate. Red and green phosphor dots can be printed and formed on flre face plate 15 by means of portions of electron beams 22b, 22b and 22c, 22c respectively in the same manner as aforementioned, so that a set of three color phosphor dots of blue, red and green can be formed for each aperture of the shadow mask. Application by a known process of the metal back 19 to the face plate having thereon a multitude of three color phosphor dots formed and printed in the manner described will provide a phosphor dot screen for a color television tube.

In one embodiment, the phosphor dots formed by the method according to this invention have a diameter of 0.3 millimeter. When no coils 29 for producing a revolving magnetic field are provided in the tube, the phosphor dots formed on the face plate have a diameter of 0.2 millimeter. It is to be noted that by finely adjusting the current flowing to the coils 29, it is possible to provide phosphor dots which has a diameter twice as large as the diameter of a converged beam spot or 0.4 millimeter.

The method according to this invention can be carried into practice with a color picture tube which is not of the after acceleration and convergence type. In this application, the voltages supplied by first, second and third power sources 24, 25 and 26 are made equal to one another. With this arrangement, the shadow mask 20 and the phosphor coat 18 on the face plate 15 are of equal potential and consequently no accelerating electric field for the electrons exists between the shadow mask 20 and the phosphor coat 18. Accordingly, the electron beam B passing through the aperture 21 of shadow mask 20 moves straight toward the phosphor coat 18 without being refracted, as shown in FIG. 12. The electron beam 22 will thus pass through the aperture 21 of shadow mask 20 and impinge on the phosphor coat 18 in the manner illustrated in FIG. 1B. In this instance, the electron beam is not converged as is the case with the embodiment illustrated in FIG. 3. If the electron beam 22 is deflected by the coils 29 for producing a revolving magnetic field, phosphor dots larger in size than the electron beam spots 27 can be formed on the face plate 15, as shown in FIG. 14.

In the embodiments described, three electron guns are arranged circumaxially about the center axis of the picture tube and disposed equiangularly at an angle of However, the method according to this invention is not limited to this form of arrangement of electron guns. Electron guns may be arranged in any way possible. For example, electron guns 36 may be arranged, as shown in FIG. 15, in a straight line in side by side relation. Further, the invention has been described with reference to embodiments for producing a phosphor dot screen for a color picture tube provided with three electron guns. It is to be understood, however, that the invention is not to be limited to this number of electron guns, and that the method according to this invention can be used for producing a phosphor dot screen for a color picture tube provided with any number of electron guns. Accordingly, the method according to this invention is not to be constructed as being limited to the embodiments described hereinabove and many modifications and combinations can be made therein without departing from the spirit of the invention.

What we claim is:

1. A method of producing a phosphor dot screen for a color picture tube comprising the steps of attaching a transparent conducting electrode to a face plate; applying a coat of a mixture of phosphor material and sensitive material to said transparent conducting electrode; attaching a shadow mask to said face plate; attaching said face plate mounting said shadow mask to an outer casing of a tube through a vacuum packing, said outer casing having a coat of conducting material applied to the inner surface thereof which serves as an anode and an electron gun assembly mounted therein, said outer casing also having deflecting coils, convergence coils and coils for producing a revolving magnetic field mounted on the outer surface thereof in such a manner that said coils for producing a revolving magnetic field are interposed between said deflecting coils and said convergence coils; evacuating the interior of the tube defined by said outer casing and said face plate; passing to said deflecting coils a standard television current for effecting vertical scanning and horizontal scanning so as to produce a deflecting magnetic field; passing two sine wave currents to said coils for producing a revolving magnetic field so as to produce a revolving magnetic field, said two currents having the same amplitude and varying in phase by 90; applying voltages of predetermined values to said transparent conducting electrode, said shadow mask and said anode, respectively; applying a voltage of predetermined value to said electron gun assembly so that electron beams may be produced by the electron gun assembly and pass through apertures of said shadow mask to strike said coat of the mixture of phosphor material and sensitive material on the face plate to thereby print on the face plate dots of electron beams which are larger in size than spots of electron beams; removing the aforementioned voltages from the electrodes and the shadow mask; restoring atmospheric pressure to said interior of the tube defined by the outer casing and the face plate; removing the face plate from the outer casing; and treating the face plate so as to wash off the portions of the coat of the mixture of phosphor material and sensitive material on which no dots are printed by the electron beams.

2. A method of producing a phosphor dot screen for a color picture tube comprising the steps of attaching a transparent conducting electrode to a face plate; applying a coat of a mixture of phosphor material and sensitive material to said transparent conducting electrode; attaching a shadow mask to said face plate; attaching said face plate mounting said shadow mask to an outer casing of a tube through a vacuum packing, said outer casing having a coat of conducting material applied to the inner surface thereof which serves as an anode and an electron gun assembly mounted therein, said outer casing also having defleeting coils, convergence coils and two sets of deflecting plates mounted on the outer surface thereof in such a manner that said deflecting plates are interposed between said deflecting coils and said convergence coils, said two sets of deflecting plates being formed of conducting material and disposed at an angle of 90 to each other; evacuating the interior of the tube defined by said outer casing and said face plate; passing to said deflecting coils a standard television current for effecting vertical and horizontal scanning so as to produce a deflecting magnetic field; applying two sine wave voltages to said two sets of deflecting plates one to each set so as to produce a revolving electric field, said two voltages having the same amplitude and varying in phase by 90; applying voltages of predetermined values to said transparent conducting electrode, said shadow mask and said anode, respectively; applying a voltage of predetermined value to said electron gun assembly so that electron beams may be produced by the electron gun assembly and pass through apertures of said shadow mask to strike said coat of the mixture of phosphor material and sensitive material on the face plate to thereby print on the face plate dots of electron beams which are larger in size than spots of electron beams; removing the aforementioned voltages from the electrodes and shadow mask; restoring atmospheric pressure to said interior of the tube defined by the outer casing and the face plate; removing the face plate from the outer casing; and treating the face plate so as to wash off the portions of the coat of the mixture of phosphor material and sensitive material on which no dots are printed by the electron beams.

3. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 1 in which said voltage of predetermined value applied to said shadow mask is lower than said voltage of predetermined value applied to said transparent conducting electrode.

4. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 1 in which said voltages of predetermined values applied to said transparent conducting electrode, said shadow mask and said anode, respectively, are identical with one another.

5. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 2 in which said voltage of predetermined value applied to said shadow mask is lower than said voltage of predetermined value applied to said transparent conducting electrode.

6. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 2 in which said voltages of predetermined values applied to said transparent conducting electrode, said shadow mask and said anode, respectively are identical with one another.

7. A method of producing a phosphor dot screen for a color picture tube having phosphor dots emitting light of a plurality of colors and electron guns corresponding in number to said plurality of colors, comprising the steps of attaching a transparent conducting electrode to a face plate; applying to said transparent conducting electrode a coat of a mixture of phosphor material of one of said plurality of colors and sensitive material; attaching a shadow mask to said face plate; attaching said face plate mounting said shadow mask to an outer casing of a tube through a vacuum packing, said outer casing having a coat of conducting material applied to the inner surface thereof which serves as an anode and electron guns mounted therein, said outer casing also having deflecting coils, convergence coils and coils for producing a revolving magnetic field mounted on the outer surface thereof in such a manner that said coils for producing a revolving magnetic field are interposed between said deflecting coils and said convergence coils; evacuating the interior of said tube defined by said outer casing and said face plate; passing to said deflecting coils a standard television current for effecting vertical scanning and horizontal scanning so as to produce a deflecting magnetic field; passing two sine wave currents to said coils for producing a revolving magnetic field so as to produce a revolving magnetic field, said two currents having the same amplitude and vary-in phase by applying voltages of predetermined values to said transparent conducting electrode, said shadow mask and said anode, respectively; applying a voltage to one of the electron guns corresponding to said phosphor material of one of said plurality of colors to actuate said electron gun while deactivating the rest of the electron guns so that an electron beam may be produced by said activated electron gun and pass through apertures of said shadow mask to strike said coat of the mixture of phosphor material and sensitive material on the face plate to thereby print dots on the face plate by the electron beam which are larger in size than spots of the electron beam; removing the aforementioned voltages from the electrodes and the shadow mask; restoring atmospheric pressure to the interior of the tube defined by the outer casing and the face plate; removing the face plate from the outer casing; treating the face plate so as to wash off the portions of the coat of the mixture of phosphor material and sensitive material on which no dots are printed by the electron beam, repeating the aforementioned cycle successively with each of the phosphor materials of the rest of the colors; and attaching a metal back to the face plate having phosphor dots of diiferent colors thereon.

8. A method of producing a phosphor dot screen for a color picture tube having phosphor dots emitting light of a plurality of colors and electron :guns corresponding in number to said plurality of colors, comprising the steps of attaching a transparent conducting electrode to a face plate; applying to said transparent conducting electrode a coat of a mixture of phosphor material of one of said plurality of colors and sensitive material; attaching a shadow mask to said face plate; attaching said face plate mounting said shadow mask to an outer casing of a tube through a vacuum packing, said outer casing having a coat of conducting material applied to the inner surface thereof which serves as an anode and electron guns mounted therein, said outer casing also having deflecting coils, convergence coils and two sets of deflecting plates mounted on the outer surface thereof in such a manner that said deflecting plates are interposed between said deflecting coils and said convergence coils, said two sets of deflecting plates being formed of conducting material and disposed at an angle of 90 to each other; evacuating the interior of said tube defined by said outer casing and said face plate; passing to said deflecting coils a standard television current for effecting vertical scanning and horizontal scanning so as to produce a deflecting magnetic field; applying two sine wave voltages to said two sets of deflecting plates one to each set so as to produce a revolving electric field, said two voltages having the same amplitude and varying in phase by 90; applying voltages of predetermined values to said transparent conducting electrode, said shadow mask and said anode, respectively; applying a voltage to one of the electron guns corresponding to said phosphor material of one of said plurality of colors to actuate said electron gun while deactivating the rest of the electron guns so that an electron beam may be produced by said activated electron gun and pass through apertures of said shadow mask to strike said coat of the mixture of phosphor material and sensitive material on the face plate to thereby print dots on the face plate by the electron beam which are larger in size than spots of the electron beam; removing the aforementioned voltages from the electrodes and the shadow mask; restoring atmospheric pressure to the interior of the tube defined by the outer casing and the face plate; removing the face plate from the outer casing; treating the face plate so as to wash off the portions of the coat of the mixture of phosphor material and sensitive material on which no dots are printed by the electron beam, repeating the aforementioned cycle successively with each of the phosphor materials of the rest of the colors; and attaching a metal back to the face plate having phosphor dots of different colors thereon.

9. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 7 in 'which said voltage of predetermined value applied to said shadow mask is lower than said voltage of predetermined value applied to said transparent conducting electrode.

10. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 7 in which said voltages of predetermined values applied to said transparent conducting electrode, said shadow mask and said anode, respectively, are identical with one another.

11. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 8 in which said voltage of predetermined 'value applied to said shadow mask is lower than said voltage of predetermined value applied to said transparent conducting electrode.

12. A method of producing a phosphor dot screen for a color picture tube as claimed in claim 8 in which said voltages of predetermined values applied to said transparent conducting electrode, said shadow mask and said anode, respectively, are identical with one another.

References Cited UNITED STATES PATENTS 3,222,172 12/ 1965 Giufirida 96-361 3,308,326 3/1967 Kaplan l17--33.S XR 3,387,975 6/1968 Tamura 117-335 ALFRED L. LEAVITT, Primary Examiner W. F. CYRON, Assistant Examiner 

